Clearly, admins would need to put their own NTP servers in their files.
Redhat defaults to having the local clock in there. Take it out. The
restrict lines above just make it so that some other system (not the
servers designated) can't adjust or even read your time. Adjust to taste.
The driftfile keeps track of how bad your local system clock is and
adjusts things at boot time. iburst sends an initial burst of time packets
to speed convergence after startup.